x-ray productionsite.iugaza.edu.ps/ghossain/files/2014/09/physnr7.pdfthe scanner (x ray machine) is...

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1212//1010//2014 122014 12::1010::32 32 PMPM Dr. maher O. ElDr. maher O. El--GhossainGhossain 11

CHAPTER CHAPTER 77MEDICAL PHYSICSMEDICAL PHYSICS


X-Ray PRODUCTION

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Two different forms of x-rays are produced -Bremsstrahlung (German for ‘braking radiation’)and characteristic radiation.Bremsstrahlung radiation results frominteractions between the electrons and the nucleus ofthe target atoms. At each interaction an x-ray isproduced, which may have an energy between zeroand a value defined by the voltage placed betweenanode and cathode. For example, a tube with 100kVp across it will produce x-rays with energiesranging from almost 0 to 100 keV.


Characteristic radiation is produced whenelectrons bombarding the target eject electronsfrom the inner orbits of the target atoms. It is calledcharacteristic because its energy is specific to thetarget element.

In the diagnosticdiagnostic rangerange mostmost xx--raysrays areare BremsstrahlungBremsstrahlung..OnlyOnly aa fewfew percentpercent ofof thethe xx--rayray energyenergy isis characteristiccharacteristicradiationradiation (eg(eg.. 1515%% atat 100100 kVp)kVp).. AboutAbout 9999%% ofof thetheelectronelectron kinetickinetic energyenergy isis convertedconverted toto thermalthermal energyenergy(heat)(heat) whichwhich mustmust bebe removedremoved fromfrom thethe tubetube.. ThereforeThereforeitit cancan bebe saidsaid thatthat thisthis typetype ofof xx--rayray productionproduction isis onlyonlyaboutabout 11%% efficientefficient..

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Principe of X-Ray PRODUCTION

interactions between the electrons and the nucleus

interactions between the electrons and electrons


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Radioactive sourcesRadioactive sources are present in the sealed and unsealed form

Unsealed sourceUnsealed radioactive substance such as liquid, powders, and gases are likely to be contained, for example within a bottle or cylinder, but may be released and manipulated when usedUnsealed sources present both external and internal hazards.

Sealed sourceRadioactive material that is (a) permanently sealed in a capsule or

(b) closely bounded and in a solid form. The capsule or material of a sealed source shall be strong enough to maintain leak tightness under the conditions of use and wear for which the source was designed, also under foreseeable mishaps. Sealed sources present only an external hazard.


Radioactive sources in medicineIn medicine there are three uses for non-sealed radioactive nuclides:

ß Biological analyses: radio-markers have been replaced progressively by non-radioactive markers.

ß Medical imaging: nuclear medicine department use radio-pharmaceuticals for diagnostics, which are ingested by the patient to obtain an image of the tissue or organ while it is functioning.

ß Therapy: radio-pharmaceuticals can constitute the treatment itself, for example iodine 131 for the treatment of thyroid cancer.

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Electric generators of ionizing radiationThe industrial applications of electric generators of ionizing radiation are neighbors of radioactive sources.

Medical applications comprise radio-diagnostics and radiotherapy:

v Medical radio-diagnostics is man’s largest source of exposure to ionizing radiation. It uses standard radiography equipment and X-scanners.

v The electric generators used in radiotherapy are electron accelerators of medium energy (20 million volts) which enable a beam of electrons or X-rays to be obtained.In the research sector the large accelerators and scientific instruments are used to study the fine structure of matter.


Effective doses from medical exposureEffective

dose (mSv)

Equivalent period of

natural radiation

RadiographyChest 0.02 3 daysPelvis 1.0 6 months

IVP 4.6 2.5 yearsBarium studies 9.0 4.5 years

CT (Chest, Abdomen)

8.0 4 years

Nuclear MedicineThyroid imaging 1.0 6 months

Bone imaging 3.6 1.8 years

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Using Ionising Radiationin Medicine


There are 3 main uses of ionising radiation in medicine:

ß Diagnosis

ß Treatment

ß Sterilisation

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ß X-Ray radiography & Fluoroscopy.ß Angiography.ß Dental X-rayß Panoramic X-rayß Mammography.ß Cardiology (Cine-Angiography.ß CT Scan.

DIAGNOSTIC RADIOLOGY


Radiation Therapy

ß Radiotherapyß Brachytherapy

Radiation therapy uses ionising radiation to treatcancer i.e. to destroy cancerous cells.

There are two techniques in radiation therapy that areused to treat cancer using ionising radiation:

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RADIOTHERAPY Brachytherapy


Nuclear MedicineIt is important to be able to study internal organs, or tissues, without the need for surgery. In such cases, radioactive tracers can be injected into the body so such studies can take place. The path of these tracers can be detected using a gamma camera because of their radioactivity.

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Sterilisationß Radiation not only kills cells, it can also kill germs

or bacteria.ß Nowadays, medical instruments (e.g. syringes) are

prepacked and then irradiation using an intensegamma ray source.ß This kills any germs or bacteria but does not

damage the syringe, nor make it radioactive.


Doctors often need to look inside our bodies without cutting them

open….

Some you may have heard of…X-rays…..CT scans…..MRI scans

And new ones you may not have heard of yet….PET scans…..MEG

are essential in the development of many scanning technologies

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Here is your chance to…ß Find out the basics of how these scans work ß See how important physics is to modern

medicine.


X-raysß Very little ordinary light can

pass through skin. It’s either absorbed at the surface or reflectedback…..

ß To “see” inside we need a kind of “light” with more energy…

Skin

Ordinary Light

X rays

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Taking “X rays”

The patient is placed in front of

a source of X RAYS

X ray Tube

A photo graphic plate is

placed on the other

side of the patient

Most of the X rays pass through the

patient’s body…….


X-rays are absorbed by bone but can pass through skin and soft

tissue

bone

Soft tissue

Photographic

plate

X rays that are absorbed in the photographic plate cause chemical changes. These show as darkened areas when the plate is developed.

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absorbed by some partlyrays are also -Xtissues in the body this creates a more subtle

picture.

bone

Soft tissue

Photographic

plate


ßWhat part of the body do these X Rays show?

Answer: A knee

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Advantages of Basic X ray Imaging

ß X rays are easy to produceß X ray machines are relatively cheapß In controlled doses X ray images are safe to

the patient


Disadvantages of X ray Imaging

ß In large doses X rays can be harmful to healthß Images are only “shadow prints” in one

“plane”…..no 3D informationß Imaging of soft tissues and organs is not

always very detailed.

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CT ScansCT or “CAT” scans take X ray imaging to

“C” stands for “Computed”“T” stands for “ Tomography”


In short….CT scanners are complex X ray machinesattached to very clever computersusing complicated mathematics to build up images of our insides.

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The patient is placed on a bedThe scanner (X ray machine) is the shape of a ring

The patient is slowly moved through the ring The patient is slowly moved through the ring as the scan takes place…as the scan takes place…


Looking end on….

X ray tube

X ray detector

Patient

X Rays are produced in an X ray tube, pass through the patient and are detected by the detector

The scanner rotates the X ray tube and detector so the patient is scanned from all angles

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Advantages of CT scans

ß Images are like “slices” ß Compared other scanners (MRI and PET)

CT machines are quite cheap.


Disadvantages of CT

ß Still use X rays that can damage healthy tissues (in large doses).ß Imaging of soft tissues is improved but still

not always as detailed as doctors require.

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MRIWhat do the letters stand for?

M……….. MagneticR………… ResonanceI…………. Imaging

.X raysuse do notMRI scanners


MRI Explained...

Your science studies have shown you that

your body is made up of living cells…

Which are made up of molecules …

Which are made up of atoms

electron

neutron

proton

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The simplest atom is…

Hydrogen1 electron

1 proton

It’s nucleus contains just one proton


In the 1940’s physicists discovered that the nuclei of some atoms have a

property called “SPIN”…….Like a wobbling spinning top.

This causes the nucleus act like a tiny magnet….

N

S

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After many years of investigation physicists found they could affect the tiny nuclear

magnets of hydrogen atoms using very strong magnets and radio waves…

Bring in the magnets….

S N

Note: The nuclear magnets can line up in two possible directions A pulse of radio waves can cause some of the nuclear

magnets absorb energy and “flip”

When this happens energy is released as a tiny pulse of radio waves !!!

….watch how the magnetic field affects the tiny nuclear magnets….watch how the magnetic field affects the tiny nuclear magnets


This tiny pulse of radio waves that can be detected and

analysed. The timing, and the energy of these signals, reveals information about

the Hydrogen atoms and what types of molecules they are

attached to.

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So what has all this got to do with looking inside your body?

ßWhat is your body mostly made of?

ßWhat is the chemical name of water?

H2OHydrogen in the most abundant element in your

body (approx 63% of all the atoms are H)


Organic molecules that make up tissues like

FATMUSCLE

TENDONS etc.

contain a large number of Hydrogen atoms

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Most MRI scanners use

magnetsAn electric current passes through a massive coil made of a special “superconducting” material

This creates a very strong magnet (x 20000 times stronger than earths magnetic field)

This may seem like a really easy way to create a strong magnet

but there is a catch……


To achieve these temperatures the superconducting coils need to sit in

a container filled with…

Thankfully the patient is insulated from this extremely low temperature whilst inside the magnet.

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The magnet used is incredibly strong!

ß Stand 1m away with a large spanner in your hand…. you would not be able to hold on to it.ß Patients have to remove all metallic objects

and credit cards…ß Patients may have metal objects inside their

bodies…


With the patient safety check complete the scan can begin…The part of the body to

be scanned is placed in the centre of the primary magnet

XThe magnet field produced has to be very steady and strongThis field causes the Hydrogen nuclei in the patients body

to line up with the field

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X

Three further coils are embedded into the tube….GRADIENT MAGNETS… these are used to fine tune the magnetic field so particular body parts and tissue types can be focused on.

The patient will know when these magnets are switched on…they can make a loud banging noise.

More coils provide a pulse of radio waves that cause some of the “nuclear magnets” to flip….

The machine waits and records any radio signals that are then emitted by the patients body…..


CT compared to MRI

ß CT scanners scan a patient in “slices” but the angle of the slice depends on how the patient is positioned in the machine.ß MRI scanners scan a whole section of the

body then the doctor can request to view a slice of the patient at any angle…ß MRI scans can reveal a lot more detail.

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“snapshot” images are usefulbut doctors sometimes need “real time”pictures of how parts of your body are

functioning…e.g. How your heart is

functioning.

Moving images can be achieved with MRI but PET scanning can give excellent results…

Introductions to PET(positron emission tomography)


PET SCANNERS LOOK LIKE CTSCANNERS…

The key differences:

-NO X RAY TUBE.-The ring is

surrounded by “Gamma Ray” detectors

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What are “gamma rays” and “positrons” ?

A little detour….


You will have heard of…ElectronsProtons

NeutronsThese are the building blocks of atoms.

particles otherPhysicists have discovered a whole host of that exist !!!

AND ASWELL: Every particle has it’s own ANTI PARTICLE…

Its….

equivalent

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The antiparticle of the electron is called a…

When an electron and a positron meet they annihilate…

The energy released creates 2 gamma rays


PET scan patients are injected with a specially created substance called a

“RADIOTRACER”….

Usually a “Radioactive” type of glucose.

The radiotracer is a source of positrons which leads to the production of gamma rays…

INSIDE THE PATIENTS BODY!

These pass through the patients body and are picked up by the

scanner.

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Looking at the scanner: end on….

Ring of gamma ray detectors

Patient

The radio tracer produces positronswhich annihilate with electrons in the patients body producing pairs of gamma rays.

The energy and position of all the gamma rays are recorded and turned into an image by a computer.


The radiotracer concentrates itself in certain tissue types…

This glucose type radiotracer has concentrated itself in high glucose using cells like the brain, kidneys and cancer cells.

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PET Scans are very expensive…

The biggest cost is in the production of the RADIOTRACERS.

The hospital needs to have access to a “CYCLOTRON” to create them

(several million euro to buy one!)

Radiotracers have to be used straight after they are produced….they cannot be stored.


Brief Introduction to MEG(magnetoencephalography)

MEG is different to all the other techniques…

It doesn’t put anything into a patients body….No X rays/Strong Magnetic Fields/Radio

waves/Radiotracers.

It just measures something that is already there…

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Your body is full of tiny electric currents!

These tiny currents produce tiny magnetic fieldsMEG is able to measure some of these.


ß MEG is mostly used to image brain activity.ß MEG scanners measure

tiny magnetic fieldsproduced by electrical brain activity.ß These fields are so

weak (10,000 milliontimes weaker that the earths magnetic field) that the scanner needs to be in a specially shielded room.

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So now you know how important PHYSICS is to MEDICINE

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